The sUAS Guide Issue 01, January 2016 | Page 33

3 - PROCESSING OF DATA

Once the requisite data was collected, the raw images and flightlog were process using Pix4D ver.2.0.100 on a Windows 10 I7 machine. 2 separate projects were processed, one with only the flightlog used to reference the images, the second using the flightlog to initially reference the images but later adding control points to further refine the model.

No-Control: 568 out of 576 images were used in the calibration. The remaining images were rejected as they appeared to be shots taken during banking by the aircraft. After processing, the absolute error found from the interpolated camera positions to computed was X:5.58m, Y:3.80 and Z:3.70. This is within the expected range given the size of the site, the delay in recording the image and instrument inaccuracies.

Control: The same results as above were computed, but ground control were added to the model and then realigned. The final RMS error for all the control was 0.02m.

Othophotos and DEM were generated for both projects and exported for analysis

4 - DATASET COMPARISON

Over 80 points where selected on each corresponding orthophoto evenly over the site. These were chosen at random so as to not be influenced or weighted by the existing control points. The points were coordinated in 2d and compared.

The overall shift from control to no-control did not appear visually to be shifted in a planar fashion with a rotation but that a scaling had also been introduced. The 2d mean shift between points averaged at dX:0.19m, dY:0.04m, however the range varied as much as rX:10.97m, rY:10.94m. The distribution of the shift is
exaggerated at the edges of the model. A Helmert transformation of the data shows that
a scaling factor of 0.995 and a rotation of 0.0619 degrees would need to be applied to
gain similar dimensions to the control data.

This information is surprisingly good as it shows that despite a rotation the linear
measurements over the entire 2km of the site are only 1% exaggerated, the
exaggeration being weighted towards the edges of the model.

The same points were compared with their respective DEM data and
Z values derived. From this information, values where expected to
have an even shift over the site as the control datum for each
survey system would have varied. The average shift was found
to be 3.01m however the range varied 13.82m. This means for
this data set, the error range is 9%. The data spread appears
to be exaggerated in the center and edges of the model.
Cross sections were drawn at random to confirm this
inconsistent shift.